This invention relates to process and apparatus for polymerizing oxygen-inhibited ultraviolet photopolymerizable resin-forming material such as a film or a deposit thereof on a substrate. For convenience herein such masses of the resin-forming material will be referred to from time to time simply as "films."
Superficial curing herein connotes polymerization of the binding vehicle in such film to convert the surface of the film from a liquid or tacky condition into a tack-free condition, detectable by touch; full curing connotes substantially complete polymerization of the film throughout its thickness to a finished state. Films that are undercured often have poor adhesion to substrates. Films with mainly a superficial cure (and somewhat undercured therebelow) frequently will develop defects such as wrinkling, "orangepeeling," or "alligatoring." Films that are well cured below, but which have a tacky or undercured surface, are generally unusable because of such tacky or undercured and thus unsatisfactory condition.
The generally advantageous ultraviolet wave energy useful for the instant process is that U.V. energy in the so-called "near-visible" or "near-U.V." region, in other words, the wavelengths of the U.V. spectrum that are transmittable through a quartz or other transparent window. Such advantageous range should be understood herein as lying between about 1600-1800 A wavelength and about 4000-4200 A wavelength, and preferably between about 2500 A and 4000 A wavelength.
Conventionally convection ovens and infrared sources have been used to cure (polymerize) films in the nature of a paint, varnish, enamel, lacquer, stain filler, ink, or adhesive. The instant film or deposit can be a clear one, optionally tinted, or an opaque one, either kind in a variety of colors for the purpose of protecting, decorating, and/or applying a message on a substrate, or for adhesively uniting a plurality of U.V. light-transmitting layers one with another. For convenience herein the film or deposit for such curing, whether it is virtually totally polymerizable, such as a binding-type vehicle by itself, or such vehicle compounded with other ingredients such as pigments and fillers, will be referred to herein from time to time as a "paint." Such paint can be a fluent, liquid phase continuous material, a jelly-like material, or a powdery mixture. It can have, if desired, opacifying pigment and/or added colorants and fillers in conjunction with such polymerizable binding vehicle. Such paint also can have various other conventional additives such as pesticides, odorants, flow-control agents, bubble breakers, defoamers, plasticizers, intercoat adhesion promoters, and other ingredients conventional in surface-coating films and adhesives.
More recently ultraviolet wave energy curing of materials has been suggested, usually using various U.V. sensitizers for sensitizing photopolymerization in ultraviolet wavelengths in the near-visible region. The literature on such photopolymerization and sensitization is abundant. Films (deposits) of paint for such U.V. curing can be quite thin, e.g., 0.1 mil or less, often are 0.5-2 mils in thickness, and can be as high as 50 or 60 mils, although heretofore such extremely thick films have often been difficult to cure. For the purpose of this specification, films (including deposits) can be continuous or discontinuous upwards to the thickness of 75 mils.
A fairly intense radiation source which emits a minor proportion of its radiation in the near-U.V. region is shown in U.S. Pat. Nos. 3,364,387 and 3,650,669; the latter patent teaches the curing of paint films by exposure to such radiation. This sustained source of U.V. energy is conveniently described as a plasma arc torch operating at essentially atmospheric pressure. It usually operates with a swirl flow. The intensity of such torch integrated throughout the entire spectrum range of its continuum light radiation can be at least about 350 watts per square centimeter steradian. Ordinarily, such apparatus is used with an inert gas atmosphere blanketing the coated workpiece to be irradiated so as to prevent inhibition of polymerization from air.
Electric lamps and lasers have been made to emit sustained U.V. radiation also for such curing purpose. An example of laser application to the cure of U.V.-polymerizable materials is in the copending U.S. patent application of Antonio de Souza and A. M. Buhoveckey, U.S. Ser. No. 189,254, filed Oct. 14, 1971. Other examples of U.V. laser and U.V. electric lamp curing of such materials are shown in U.S. Ser. No. 342,038 now abandoned of Antonio de Souza, filed Mar. 16, 1973.
Commercial electric U.V. lamps for this sustained curing purpose are shown, for example, in U.S. Pat. Nos. 3,499,781 and 3,673,140. Such commercial lamps are usually referred to as "mercury U.V. lamps" even though they can have gases mixed with the mercury which modify the emanations. They ordinarily have medium filling pressure. U.V. light sources for flash photolysis also can be conventional and devised on principles related to the sustained electrical U.V. photolysis light sources except that they are controlled to emit short bursts or flashes of energy interrupted by down time. A typical flash photolysis source is shown in West German patent No. 2,019,270 of Nov. 16, 1972. The subject matter of these patents and all the patent applications cited herein is expressly incorporated herein by reference.
Molecular oxygen in the atmosphere surrounding the film usually is inhibitory to the full curing of otherwise U.V.-photopolymerizable resin-forming masses. In such instance often the surface in contact with such atmosphere remains undercured. Additionally, any ozone present is especially so inhibiting. Hence the masses to be cured usually are protected from air atmosphere with nitrogen or other inert blanketing gas at considerable expense and trouble. Such inhibiting atmosphere also can be substantially more or less rich in molecular oxygen than air is and still be quite inhibitory to satisfactory curing of the film or deposit.
Advantages of U.V. curing over other conventional curing in ovens or the like include especially the ability of the U.V. irradiation to perform "cold" polymerization ("curing") of the film at high speed with attendant suppression of losses due to volatilization of components of the film, suppression of discoloration or degradation of the film, suppression of shrinkage and distortion of the film and of the substrate (preservation of dimensional stability), and suppression of degradation of the substrate to which the film is applied, particularly when such substrate is plastic, paper, or fabric.
Special advantages of the instant invention include effective U.V. curing of the films or deposits in economical atmospheres such as air, a lessened cooling load on the individual U.V. light sources of the apparatus, and the ability to distribute such load over a plurality of U.V. light sources. The sustained operation of U.V. lamps at about their highest intensity often overheats them and causes production slowdowns. This invention permits, in many cases, beneficial reduction of energy intensity from the sustained irradiating U.V. light source or sources used according to invention principles.